Refractory resistance and method of manufacture

ABSTRACT

An electric igniter for gas burners and the like having a selfresistant wire coated with an insulating material encased in a conductive coating of material having a high melting point, optimum coefficient of thermal resistivity and resistivity to oxidation at high temperature.

UNITED STATES PATENTS 3.323201 6/196'7 sc e ip em 1 g g O United States Patent 91 111.1 3,810,734

Wills0n a 1451 May 14,- 1974 [54] REFRACTORY RESISTANCE AND METHOD 3,393,038 7/1968 Burkhalter et al 431/66 OF TU E 3,468,218 3/1972 Kellerman 338/296 X v 3,521,213 7/1970 Hardy 219/270 x [76] Inventor: James R. Willson, 94 Golden Hill Rd., Trumbull, Conn. 066l l r I Primary Examiner-Edward G. Favors [22] Sept 1971 Attorney, Agent, or Firm-Anthony OBrien 21 Appl. No.2 179,371 I Related US. Application Data {63] Continuation-in-part of Ser. No. 803,334, Feb. 28, ABSTRACT v 3 I969, abandoned.

An electric igniter for gas burners and the like having [52] US. Cl. 431/258, 219/270 a'self-resistant wire coated with anins ulating material [51] Int. Cl. F23q 7/22 1 encased in a conductive coating of material. having a [58] Field of Search 431/258, 66, 67; 317/94, high melting point, optimum coefficient of thermal re- 3l7/98; 219/270 X; 338/296 sistivity and resistivity to oxidation at high tempera- 1 ture.

[56] References Cited 2 Claims, 5- Drawing Figures REFRACTORY RESISTANCE AND METHOD OF ..-MA U.EAQTL BE .t.

.The present application is a continuation-in-part of my application Ser. No. 803,334 filed Feb. 28, 1969,

now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to electric igniter constructions for fuel burners of the type disclosed in my US. Pat. Nos. 3,472,220 issued Oct. 14, 1969 and 3,493,174 issued Feb. 3, 1970 and particularly to such constructions having high temperature resistance to oxidation and improved mechanical strength.

Materials particularly well-suited for use as gas burner igniters, due to their high melting points, good resistance to oxidation at high temperatures, resistivity and coefficient of thermal resistivity, are well-known in the prior art. Amongsuch materials are molybdenum disilicide (MoSi tungsten disilicide (WSi and sili con carbide. Materials falling in thisgroup, however,

generally exhibit poor structural strengthand are easily mechanical properties, however, are unsuitable because of poor resistivity to oxidation at high tempera- I tures.

SUMMARY OF THE INVENTION An object of the present invention is to provide an electric igniter construction for fuel burners which overcomes the disadvantages of the prior art by furnishing a composite igniter formed of materials which variously provide good high temperature resistivity to oxidationand suitable mechanical strength.

The present invention is embodied in a refractory resistance and method of making wherein a wire core is coated with an insulating material and encased in a conducting coating of material selected from the group consisting of molybdenum disilicide, tungsten disilicide and silicon carbide.

These and other objects and advantages of the present invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of an igniter constructed in accordance with the invention;

FIG. 2 is an elevational view of an igniter construction in accordance with the invention during an initial stageof fabrication thereof;

FIG. 3 is a view similar to FIG. 2 showing the igniter in a subsequent stage of fabrication;

' FIG. 4 is a view similar to FIG. 2 showing the igniter substantially at the final stage of fabrication; and

FIG. 5 is a partial schematic view showing the positioning of an electic igniter relative to a gas burner and the fuel flow control therefor.

' DESCRIPTION OF THE PREFERRED EMBODIMENT An igniter for fuel burners constructed in accordance with the present invention is shown in FIG. 1 and includes a U-shaped igniter coil, shown generally at 10, electrically connected at the ends thereof to a pair of leads or Qfi .F ....l. h h e. n. EEK" "stu through an insulating base 14. Connection between at least a conducting coating 26 (FIG. 4) on the ends of the coil 10 and thecontacts 12 may be made by braz-' ing, as is known in the art, or by any other suitable means of mechanical interconnection. The base 14 is preferably formed of a ceramic material cast around the leads 12 as is also known in the art.

With particular reference to FIG. 2, the coil is shown during a stage of fabrication thereof and comprises a self-resistant wire coil loop 16 fomied of tungsten or the like. The loop is mounted in a chemical vapor deposition furnace 18 by electrical holders 20 which are in turn connected to a suitable source of electric current by means of conductors 22. For the purposes of this invention, the wire 16 can comprise other suitable materials having electrical and mechanical properties similar to tungsten such as, for example, Titanium, Manganese or Nichrome. Y

In FIG. 3, the coil-10 is shown during a subsequent stage of fabrication. With current applied to the loop 16 to induce a self-heating, a coating of insulating material 24, such as alumina or the like, is deposited on the loop by chemical vapor desposition in a manner well-known in the art. The insulation 24 can comprise other refractory materials having electrical properties similar to that of alumina such as, for example, mullite, steatite, or cordierite.

In addition to the chemical vapor deposition method of applying the insulation, other methods such as slip casting orthe like may be utilized to form the insulating coating if desired.

In FIG. 4, the conducting coating 26 of molybdenum disilicide or the like is deposited on the insulating coating 24 by chemical vapor deposition. The insulating material prevents the conducting coating 26 from shunting the loop 16 during the deposition of the conducting coating. Other refractory conducting coatings such as, for example, tungsten disilicide or silicon carbide, having high melting points, good resistance to oxidation at high temperature, resistivity and coefficient of thermal resistivity, according to the following table, are suitable for forming the conducting coating 26:

Electrical Resistivity: 21.5 X 10' OHM-CM Coefficient of Thermal Resistivity: Positive The materials suitable for the coating 26 should furthermore be resistant to oxidation at operating temperatures above 2, 00O F, i

The coilll) is then removed from the holders 20 and the free ends of the wire 16 are trimmed flush with the ends of the coatings 24 and 26. At least the conducting coating 26 of the coil 10 is then attached to the contacts 12 and the assembly is completed, as shown in FIG. I, by mounting the contacts in insulating base 14.

The loop 16 is more resistant and less conductive than the conducting coating 26 and provides additional mechanical strength to the coil 10 to resist mechanical load or shock.

For a specific example of the present invention, a

It should be clear that fuel burner igniters having forms other than those illustrated such as, for example, coils, straight wires or the like, could be fabricated in accordance with this invention. The base 14 of the igniter could also be fabricated in various forms without altering the intent of the invention. There are also many materials and combinations of materials that could serve the purposes of this invention and the choice thereof would primarily be dicated by cost and capability of fabricating the materials in anacceptable form.

Factorsdictating acceptable form are coil size in the range of an inch in length and one quarter inch in diameter and the cross sectional areas which will result in reasonable resistance to provide sufficiently high operating voltages (greater than 2.5 volts and preferably at least 24 volts). Another consideration in material selection is that they are both chemically and physically compatible with one another in the environment of the igniter, namely, in close proximity to a high heat burner.

As is illustrated in FIG. 5, the electric igniter is positioned within igniting proximity of a gas burner 28 which is connected to a source of fuel gas (not shown) by conduits 32 and 34. The flow of fuel gas is controlled by any suitable control device 30, such as an electromagnetic valve Wires 36 from the electric igniter 10 are suitably connected to the control device 30 and its power source (not shown) to assure energization of the igniter prior to opening of the electromagnetic valve.

Inasmuch as the present invention is subject to many variations and changes in detail, it is intended that all an electrical refractory insulating coating of alumina encasing said wire core, said insulating coating having a thickness in the range from 0.10 to 0.20 inches,

a conductive refractory coating encasing said insulating material, said conductive refractory coat ing having a thickness in the range from 0.005 to 0.015 inches,

a pair of leads connected to the conductive refractory coating at respective ends of the igniter, said conductive coating being selected from the group consisting of molybdenum disilicide, tungsten disilicide and silicon carbide, and

said conductive coating being more conductive than said wire core.

2. The invention as recited in claim 1 wherein said wire core is formed of tungsten. 

2. The invention as recited in claim 1 wherein said wire core is formed of tungsten. 